The molecular process of viral fusion, in which viral coat proteins recognize and bind to surface receptors of the host cell, is a critical target in the prevention and treatment of viral infections. Upon recognition of the viral glycoprotein by host cellular receptors, viral fusion proteins undergo conformational changes that are essential to viral fusion and infection. A series of hydrophobic amino acids, located at the N- and C-termini organize to form a complex that pierces the host cell membrane. Adjacent viral glycoproteins containing two amphipathic heptad repeat domains fold back on each other to form a trimer of hairpins, consisting of a bundle of six α-helices which is referred to as a spike. Each of the glycoproteins of the trimer is tethered to the viral surface by a membrane-proximal ectodomain region (MPER). This six-helix bundle motif is highly conserved among many viral families, including Filovirus (ebola), (Malashkevich, V. N., et al., PNAS, 1999. 96: 2662-2667; Weissenhorn, W., et al., Molecular Cell, 1998. 2(5): p. 605-616), Orthomyxovirus (influenza) (Wilson, I. A., J. J. Skehel, and D. C. Wiley, Nature, 1981. 289: 366-37; Bullough, P. A., et al., Nature, 1994. 371(6492): p. 37-43), Coronavirus (SARS) (Xu, Y. H., et al. Journal of Biological Chemistry, 2004. 279: 49414-49419), Paramyxovirus (HRSV) (Zhao, X., et al., PNAS, 2000. 97: 14172-14177) and Retrovirus (HIV) (Chan, D. C., et al., Cell, 1997. 89: 263-27; Weissenhorn, W., et al., Nature, 1997. 387: 426-430).
Vaccines can provide an effective method to prevent viral infection. However, selection and/or generation of an appropriate viral antigen is not a trivial undertaking. The challenge of vaccine development is especially difficult for the prevention of infection by viruses with greater structural diversity and/or that undergo rapid mutation. Substantial challenges to vaccine development arise from many aspects of HIV-1 biology including viral sequence diversity of HIV proteins of which the virion surface gp160 spike protein is an example (Korber, B., et al. 2001. Evolutionary and immunological implications of contemporary HIV-1 variation. Br. Med. Bull. 58, 19-42). As in the general viral fusion steps described above, gp160 is synthesized as a precursor, cleaved by furin-like enzymes in the trans golgi into gp120 and gp41 subunits that noncovalently associate, and assembled into heterotrimers. gp120 binds to cell-surface CD4, then undergoes conformational change revealing a coreceptor attachment site (Feng et al., 1996. HIV-1 entry co-factor: functional cDNA cloning of a seven-transmembrane, G protein coupled receptor. Science 272, 872-877) whose ligation in turn induces structural rearrangements within the transmembrane gp41 subunit to fuse viral and host cell membranes (Chan et al., 1997. Core structure of gp41 from the HIV envelope glycoprotein. Cell 89, 263-273.). gp160 is extensively glycosylated, displays prominent variable loop segments, exists in several conformational states, and is proteolytically labile. As a result antibody responses to HIV tend to be strain specific, and vaccines to such epitopes are not substantially useful in the prevention of HIV infection.
Naturally occurring viral cross-species neutralizing antibodies are rarely elicited against conserved structural elements that are typically shielded, difficult to access, or transient, in HIV infection as well as other viral infections. Not surprisingly, only a handful of human broadly neutralizing antibodies (BNAbs) have been identified to date against HIV (reviewed in Douek et al., 2006. The rational design of an AIDS vaccine. Cell 124, 677-681). Those BNAbs with the greatest viral clade and strain breadth, including the monoclonal antibodies 2F5 and 4E10 (derived from immortalized B cells of HIV-1-infected individuals) and Z13e1 (selected from an affinity-matured phage display library using bone marrow RNA derived from a clade B-infected individual), each targets the membrane-proximal ectodomain region (MPER) of gp41 (Nelson et al., An affinity enhanced neutralizing antibody against the MPER of human immunodeficiency virus type 1 (HIV-1) gp41 recognizes an epitope between those of 2F5 and 4E10. J. Virol. 81, 4033-4043). The MPER, although accessible to antibody, rarely, if ever, elicits BNAbs during natural infection.